In a typical radio communications system, radio communications terminals, referred to as radio terminals or user equipment terminals UEs, communicate via an access network with other networks like the Internet. For example, a radio access network (RAN) in a cellular communications system covers a geographical area which is divided into coverage cells, with each cell being served by a base station, e.g., a radio base station (RBS), which in some networks is also called a “NodeB” or an enhanced Node B “eNodeB.” Each base station typically serves several cells. One common deployment is 3-cell base station installations, where a base station serves three cells. Other wireless systems, like WiFi systems, employ access points (APs) to provide network access to wireless terminals. For simplicity, wireless access points, radio base stations, and the like are referred to generally as base stations and user equipment terminals, access terminals, and the like are referred to generally as radio terminals.
A base station communicates over the air interface operating on radio frequencies with the radio terminals within range of the base stations. The radio signals may either be dedicated signals to and from specific radio terminals, multicast signals intended for a subset of the radio terminals in a cell or coverage area, or broadcast signals from the base station to all radio terminals in a cell or coverage area. For simplicity, a cell is understood to include a radio coverage area or the like. A base station broadcasts information to all the radio terminals in a cell using the broadcast channel of the serving cell. Each cell is identified by a cell identifier within the local radio area, which is broadcast in the cell.
Small scale base stations have recently been introduced that are connected to broadband internet service and provide coverage for very small areas sometimes called femto cells. Femto cells are similar to WiFi “hotspots” but are part of a cellular network rather than a wireless local area network (WLAN). The femto base stations work in many ways like a larger “macro” base station would, but on a much smaller scale with low output power designed for small spaces such as apartments, houses, offices, etc. A pico base station is a “small” base station, and a femto base station (e.g., a home base station) may be even smaller. However, for purposes of this application, a femto base station includes any small or very small base station that is distinguishable from a macro base station. Femto base stations provide a better signal in smaller interior or closed spaces where signal quality between regular macro base stations and mobile phones is poor due to the proximity of macro base station towers or just due to the material of the building or other obstructions blocking the signal. Instead of using a traditional base station for access, the radio terminal gains access through the femto base station to gain access to the IP access network.
FIG. 1 shows an example of a cellular communications system that includes a small scale femto base station and a traditional macro base station. A first building 1 includes a radio terminal 2 that receives radio signals from a macro base station 3. The macro base station 3 is coupled to a core network 5 either directly or through a radio access network 4. The core network 5 provides access to the Internet 6 and other networks. A second building 7 includes another radio terminal 8 that receives radio signals from a femto base station 9. The femto base station 9 may be coupled, typically via some broadband access mechanism (wired or wireless), to the core network 5 either directly or through a radio access network 4. Again, the core network 5 provides access to the Internet 6 and other networks. Because the femto base station is located inside the building 7 and is typically only intended to provide coverage within and in close proximity to the building 7, its transmit power can be considerably lower than that of the macro base station 3, which has a much larger and varied coverage area, while still providing high data rate service. Cells managed by macro and femto base stations all require cell identifiers. As the number of macro and femto cells increases, managing cell identifiers becomes more complex.
In a conventional wireless network, each cell is assigned a long identifier which may be referred to as, for example, a global cell identifier (“GCI”), a sector identifier (“SectorID”), an access node identifier (“ANID”), or some other type of identifier. Additionally, each cell may be assigned a short identifier, which may be referred to as, for example, a physical cell identifier (“PCI”), a pilot pseudorandom number (“PilotPN”), or as some other type of identifier. The short identifier, referred generally hereafter as PCI, may be used to modulate physical layer channels and is also used in neighboring cell measurements and measurement reporting by the radio terminal.
Current cellular radio systems include for example Third Generation (3G) Universal Mobile Telecommunications System (UMTS) operating using Wideband Code Division Multiple Access (WCDMA) and Fourth generation (4G) systems, like the Long Term Evolution (LTE) of UMTS operating using Orthogonal Frequency Division Multiple Access (OFDMA). One important focus area in the LTE and System Architecture Evolution (SAE) standardization work is to ensure that the new network is simple to deploy and cost efficient to operate. The vision is that the new system will be self-optimizing and self-configuring in as many aspects as possible. Such self-management is challenging with regard to cell identifiers as the number of macro and femto cells increases. One particular challenge is “collisions” between two cells using the same cell identifier.
The total number of different PCIs is typically limited. Consequently, it is desirable for a network operator to ensure that the same PCI is not used by cells that are relatively close to each other to avoid collisions between communications of neighboring cells. This also means that the PCI for a macrocell in a specific location is unique and sufficient to identify the macrocell. Even with network planning, PCI allocation is a demanding task to ensure that neighboring macro cells do not use the same PCI. But this is not feasible in an unplanned or ad-hoc network employing many small-coverage cells points. In an ad-hoc network, a network operator or a customer may deploy a base station without knowing which PCI should be used to ensure that collisions do not occur (if collisions are indeed entirely avoidable).
3GPP has standardized PCI selection algorithms for femto or home base stations referred as HeNBs in TS 36.300. The base station, referred to as an eNB, bases the selection of its PCI either on a centralized or distributed PCI assignment algorithm. For centralized PCI assignment, an operations and maintenance node (OAM) signals a specific PCI value which is selected by the eNB as its PCI. For distributed PCI assignment, the OAM signals a list of PCI values to the eNBs. An eNB may restrict this list by removing PCIs that are: a) reported by UEs; b) reported over the X2 interface by neighboring eNBs; and/or c) acquired through other implementation dependent methods, e.g., heard over the air using a downlink receiver. The eNB selects a PCI value randomly from the remaining list of PCIs.
The problem with this approach is that it may not work, particularly in heterogeneous networks with many macro and/or femto cells. A centralized PCI selection algorithm relies on the OAM to provide a single PCI value that does not collide with any neighbor cells. Although this is possible to achieve for macro cell deployment, based on careful cell planning, as mentioned above, it is not possible in certain situations such as a heterogenous network of HeNBs or other ad hoc networks. Another issue is that the exact location of an HeNB is difficult to determine, especially in the vertical axis, e.g., in an apartment building. Even if the HeNBs report detailed radio measurements to an OAM, those radio measurements can change very quickly. Indeed, the HeNB may be moved, e.g., from under a table to the window sill, potentially creating a new PCI collision.
The distributed PCI selection algorithm takes local information into account so that the HeNB can choose its PCI. In this approach, neighboring PCIs are reported by UEs, but a HeNB does not have any associated UEs until it starts transmitting making this approach ineffective during initial PCI selection. Even if a temporary initial PCI is used, a HeNB likely only has a few associated UEs, and their reports may not provide a 360 degree view of PCIs being transmitted by nearby base stations. Although some HeNBs may be able to detect PCIs being transmitted by some nearby base stations, the range of an HeNB receiver is usually limited.
Given the high probability of PCI collisions in the situations outlined above, notwithstanding the standardized PCI selection approaches, there is a need for a more effective technique for detecting and resolving collisions in wireless networks.